Liquid crystals are used, in particular, as dielectrics in display devices since the optical properties of the liquid crystals can be affected by an applied voltage. Electrooptical devices based on liquid crystals are extremely well known to those skilled in the art and may be based on various effects. Devices of this type are, for example, cells having dynamic scattering, DAP (deformation of aligned phases) cells, guest/host cells, TN cells having a twisted nematic structure, STN (supertwisted nematic) cells, SBE (super-birefringence effect) cells and OMI (optical mode interference) cells. The most common display devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
The liquid-crystal materials must have good chemical and thermal stability and good stability toward electrical fields and electromagnetic radiation, especially in the visible and ultraviolet spectral range. Furthermore, the liquid-crystal materials should have low viscosity and give short addressing times, low threshold voltages, high contrast in the cells and at the same time a wide viewing angle. This wide viewing angle results from the so-called first "minimum" condition (U.S. Pat. No. 4,398,803). The optical path difference, i.e., the product of cell thickness and birefringence, is to be adjusted to about 0.4-0.5 .mu.m. Therefore, for a cell thickness of about 2.5-4 .mu.m, liquid-crystalline mixtures with birefringence of about 0.12 to 0.19 are required. Furthermore, they should have a suitable mesophase, for example, for the above mentioned cells, a nematic or cholesteric mesophase, at customary operating temperatures, i.e., generally in the broadest possible range above and below room temperature. Since liquid crystals are generally used as mixtures of a plurality of components, it is important that the components are readily miscible with one another. Further properties, such as electrical conductivity, dielectric anisotropy and optical anisotropy, must meet various requirements depending on the cell type and the area of application. For example, materials for cells having a twisted nematic structure should have positive dielectric anisotropy and low electrical conductivity.
For display devices addressed with low multiplex ratios (common use displays), which are the preferred subject-matter of the present invention,nematic mixtures having clearing points above 55.degree. (preferably above 65.degree.), birefringence in the range from 0.165 to 0.130 (preferably 0.185 to 0.120) and threshold voltages in the range from 1.20 to 2.20 Volts (preferably from 1.40 to 1.90) are desired.
A known mixture from the prior art is E80A (BDH, Poole, Great Britain), which contains cyanobiphenyls and has the following ethyl-parameters:
T.sub.S,N =-15.degree. PA0 T.sub.N,I =60.0.degree. PA0 .DELTA.n=0.1460 PA0 .eta..sub.20 =41 cSt PA0 V.sub.10 (TN)=2.07 volts
For many applications, however, mixtures of this type have undesired low birefringence and/or a nematic phase range which is too small for outdoor applications.
It is an object of the invention to provide liquid crystalline mixtures which overcome the inadequacies of the prior art, such as those discussed above.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.